在类中调用函数，而不是在Python中使用多重处理调用函数

在有关多处理的Python文档中，有许多使函数工作并行化的示例。我以为也可以对一个类中包含的函数执行此操作。但是，以下示例不起作用。它生成用于计算当前进程号的2的乘积的进程。报告对象内部的计算值是可行的，但是，当我尝试在作业完成后获取计算值时，它只会报告返回在构造函数中设置的值。

类定义

import multiprocessing


class MyClass():

    def __init__(self,runname):
        self.runname = runname
        self.output = 0



    def calculate(self,input):
        self.output = input*2
        print "Reporting from runname %s, calculation yielded %s" % (self.runname,self.output)


    def getOutput(self):
        return self.output

调用对象的代码：

objectList = [] #Store objects
jobList = [] #Store multiprocessing objects

#Run the workers in  4 parallel processes
for i in range(4):

    thisRunname = 'Worker:%s' % i
    thisInstance = MyClass(thisRunname)

    p = multiprocessing.Process(target=thisInstance.calculate, args=(i,))

    jobList.append(p)
    p.start()
    objectList.append(thisInstance)

for thisJob in jobList: #Wait till all jobs are done
    thisJob.join()

print "Jobs finished"

for thisInstance in objectList:
    print "Worker %s calculated %s " % (thisInstance.runname,thisInstance.getOutput() )

输出：

Reporting from runname Worker:0, calculation yielded 0
Reporting from runname Worker:1, calculation yielded 2
Reporting from runname Worker:2, calculation yielded 4
Reporting from runname Worker:3, calculation yielded 6
Jobs finished
Worker Worker:0 calculated 0
Worker Worker:1 calculated 0
Worker Worker:2 calculated 0
Worker Worker:3 calculated 0

因此，可以毫无问题地生成calculate函数，当尝试检索计算出的值时，它只返回0（即在构造函数中设置的值）。

我是否缺少一个关键概念，如何获得self.output值？